A glass substrate has hitherto been widely used as a substrate for a flat panel display such as a liquid crystal display, a hard disk, a filter, a sensor, or the like. In recent years, in addition to a conventional liquid crystal display, an OLED display has been actively developed by virtue of its self-luminescence, high color reproducibility, wide viewing angle, high-speed response, high definition, and the like. Some of the developed OLED displays have already been put to practical use. Meanwhile, a display for a mobile device such as a smartphone requires a screen with ultra-high definition, because such display has a small area but needs to display a lot of information. Besides, such display requires high-speed response as well, because a moving image is displayed thereon.
In such application, an OLED display or a liquid crystal display driven by a LTPS is suitable. The OLED display emits light when an electric current flows through an OLED element serving as a constituent of a pixel. Therefore, a material exhibiting low resistance and a high electron mobility is used as a drive TFT element. As such material, an oxide TFT formed typically of indium gallium zinc oxide (IGZO) has attracted attention, aside from the above-mentioned LTPS. The oxide TFT has low resistance, a high mobility, and can be formed at relatively low temperature. The conventional p-Si.TFT, in particular the LTPS, is liable to have variations in TFT characteristics upon its formation on a large-size glass substrate, owing to instability of an excimer laser to be used in polycrystallization of an amorphous Si (a-Si) film. Therefore, in a TV application or the like, display unevenness is liable to occur in a screen. In contrast, the oxide TFT is excellent in homogeneity of TFT characteristics upon its formation on a large-size glass substrate. Therefore, the oxide TFT has attracted attention as a potential TFT formation material, and some of the oxide TFTs have already been put to practical use.